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I do see from the docs that:

By default, empty groups are omitted, and non-empty groups are generated in ascending (natural) order.

But I can’t find where it says how to change this…

I don’t think there’s a way to do this currently. Setting filter: null works for the bin transform because the domain is quantitative or temporal, so we know where the missing values are. The same isn’t true with an ordinal domain for the group transform; the transform can only see values that exist and are therefore non-empty. I think we could do this if we allowed the group transform to take a domain option, enumerating all the expected (and allowed) values; the group transform would ignore anything outside the domain. We could perhaps also allow the interval option to compute the domain from a continuous extent (e.g., days or integers). You’re welcome to file a feature request, or I can try to remember to do that tomorrow. Thanks for the suggestion!

@Mike Bostock Would you prefer the feature request in the GitHub Issues, or somewhere else? Happy to do this however you’d like.

🙂 Hi Alex! Thank you for surfacing this issue. It would be terrific if you could add it to our feedback repo: https://github.com/observablehq/feedback/issues It's always great for us to read as much motivating and clarifying context as you'd like to offer. ❤️

For Plot specifically I would prefer here https://github.com/observablehq/plot/issues but if it involves the Observable platform more generally then the feedback repo as Aaron suggested.

oop

Thanks for filing the issue! 🙏

Thank you also for refiling! Apologies for initially leading you astray 🤦